This invention in general relates to devices by which optical fibers can be optically coupled one to another and more particularly to apparatus by which expanded-beam type connectors can be fabricated and used with relaxed tolerances compared to what the tolerances would otherwise have to be to achieve similar throughput between fibers.
As is well-known, fiber optical links have the same basic elements found in electrical communication systems. Electrical signals are converted into light signals which are transmitted through optical fibers to a receiver where light signals are converted back to electrical ones from which information is derived. In the link, connectors serve to assure that tight physical or optical contact is made and maintained between the optical fibers and the transmitting or receiving components of the system.
Although seemingly simple, making connections between the components of a fiber optic link in which optical fibers of hairlike dimensions are employed is extremely troublesome and very different from making an electrical connection which requires only a reliable physical contact between two conductors. For example, a proper connection between optical fibers requires that the ends of the fibers be accurately angularly, laterally, and longitudinally aligned to assure that light leaves and enters them within a certain range of angles. If not, leakage occurs causing large signal losses which make an otherwise attractive communication link impractical. To solve the connector problem with tolerable losses, those skilled in the art have developed a class of connectors referred to as expanded-beam or imaging type connectors which are of the sort described in, for example, U.S. Pat. Nos. 4,183,618 and 4,186,995 and in an article entitled, "Connectors that stretch" appearing in the October 1980 issue of Optical Spectra.
The essence of the expanded-beam type connector is to enlarge and collimate or roughly collimate the light beam which emerges from the input fiber or fibers which are accurately placed in one half of the connector at or nearly at the focus of its lens. The other half of the connector, similar in design to the first half but which may in fact be scaled to be larger, then acts optically in reverse to the first connector half by accepting the expanded beam from the first connector half and focusing it into the output fiber end located at or nearly at the axial focus of the other connector half. In this manner, the task of optical alignment becomes one of mechanically aligning relatively large beam cross sections rather than small fiber ends as is done in strictly mechanical or butt-type connectors. With such connectors, however, the burden on optical performance and related mechanical geometry is great and must be maintained to a high degree of precision integrated into the connector to assure that the connector itself does not create high losses. For example, the permissible angular tilt between the lens surfaces of such connectors must be maintained to tolerances on the order of tenths of a degree if losses are not to exceed 0.5 db.
This invention has for one of its objects the provision of apparatus by which the tolerances for such expanded-beam type connectors can be relaxed, particularly the angular tilt of their refracting surfaces, through the use of holographic optical elements.
Those skilled in the art have made use of holographic techniques and holographic optical elements in fiber optic environments as is evidenced by the multiplexer described in U.S. Pat. No. 4,359,259. Thus, it is known that the properties of holographic optical elements can be exploited in a useful way to solve some of the problems associated with fiber optic communication systems.
Holographic optical elements differ from conventional ones because they operate by diffraction rather than by refraction, as in the case of lenses and reflection as with mirrors. They are like diffraction gratings in that they deflect light of different wavelengths at different angles but are free from the limitations of gratings because they can focus, defocus and collimate light as well, i.e., they have lens-like properties.
Another useful property of holograms, both transmissive and reflective, is that subsequent illumination by one of the wave fronts used in constructing them results in the reconstruction of the other wave front.
Although these properties and characteristics are known, they have by no means been fully exploited in applications involving optical fiber systems. Consequently, it is another object of the present invention to provide a holographic optical element for use in optically coupling a plurality of optical fibers in various predetermined combinations.
It is another object of the present invention to provide optical apparatus for coupling a plurality of optical fibers in predetermined ways.
It is another object of the present invention to provide a multiplexer/demultiplexer for use with a plurality of optical fibers.
It is yet another object of the present invention to provide a method by which optical coupling components for use with optical fibers can be fabricated.
It is still another object of the present invention to provide holographic optical elements for use in conjunction with expanded-beam type optical connectors in such a way that the fabrication tolerances heretofore required in fabricating such connectors can be relaxed.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention, accordingly, comprises the apparatus possessing the construction, combination of elements, and arrangement of parts, and the methods, which are exemplified in the following detailed disclosure.